The second hallmark of superconductivity, the disappearance of thermoelectric effects, was discovered by Walter Meissner in 1927 [1]}. Lack of thermo-e.m.f. suggests that entropy of the S fraction of conduction electrons (called “superconducting electrons") is zero. If so, then superconductivity can be a previously unknown thermodynamic state of matter characterized by complete ordering of superconducting electrons. Gorter and Casimir used zero entropy as the base postulate of their two-fluid model (1934) [2]}, a seminal thermodynamic theory addressing properties of superconductors in zero field. On the other hand, a non-zero thermo-e.m.f. looks incompatible with zero resistivity of the S state, since otherwise it would lead to an infinite current and therefore to an instantaneous restoration of the N state.
